A pipeline laid on the bottom of the sea allows a high-pressure fluid to flow therein. The pipeline is further subjected to repeated distortion caused by waves and to a seawater pressure. Therefore, a steel pipe used for the pipeline on the bottom of the sea is required to have high strength and high toughness.
If the wall thickness of a seamless steel pipe for line pipe is increased, the high strength can be achieved. However, the increase in wall thickness is liable to cause brittle fracture and decreases the toughness. Therefore, a seamless steel pipe for line pipe used on the bottom of the sea is especially required to have excellent toughness.
A method for manufacturing a seamless steel pipe for line pipe while improving the toughness has been disclosed in JP2000-104117A (Patent Document 1). In the manufacturing method disclosed in Patent Document 1, the steel pipe temperature immediately after piercing-rolling is at least 950° C., and the steel pipe is soaked at a temperature of 900 to 1000° C. with the steel pipe temperature maintained above the Ar3 point. Then, the soaked steel pipe is cooled at a cooling rate of at least 5° C./sec.
Also, methods for manufacturing a steel pipe other than the seamless steel pipe for line pipe while improving the toughness have been disclosed in JP63-215309A (Patent Document 2), JP9-3539A (Patent Document 3), JP2008-266700A (Patent Document 4), JP3755163B (Patent Document 5), and JP3855300B (Patent Document 6).
In the manufacturing method disclosed in Patent Document 2, a piercer, a mandrel mill, a cooling apparatus, a reheating furnace, and a stretch reducer are used. A billet is pierced by the piercer to produce a hollow shell, and the hollow shell is elongated and rolled by the mandrel mill. Then, the elongated and rolled hollow shell is cooled to a temperature of at most the Ar1 point by the cooling apparatus, and the cooled material pipe is sized by the stretch reducer.
In the manufacturing method disclosed in Patent Document 3, a finish-rolled steel pipe is cooled from a temperature of at least the Ar3 point at a cooling rate higher than that of air cooling. The cooled steel pipe is tempered at a temperature of at most the Ac1 point.
In the manufacturing method disclosed in Patent Document 4, a sized steel pipe is acceleratedly cooled. The acceleratedly cooled steel pipe is held at a temperature of 350 to 600° C.
In the manufacturing method disclosed in Patent Document 5, a finish-rolled steel pipe is heated to a temperature of 850 to 1100° C., and the heated steel pipe is quenched. The cooling rate for quenching is not subject to any limitation.
In the manufacturing method disclosed in Patent Document 6, a finish-rolled steel pipe is cooled to a temperature of at most the Ar3 point at a cooling rate of at least 80° C./sec, and the cooled steel pipe is quenched and tempered.